Merge branch 'acquire-before-asking-for-build' into 'master'

Acquire before asking for build

This introduces a three fixes:
- Acquire runner context before build, instead of locking main queue,
- Give back the runner to main queue, making it faster for the worker threads to pick new builds,
- Execute `docker-machine ls` only once at time if more than one goroutine asks for it.


See merge request !200